Today carbon fibers are produced chiefly from polyacrylonitrile (hereinafter referred to as "PAN") and pitch as starting materials.
PAN carbon fibers having a tensile strength of at least 300 kg/mm.sup.2 are commercially available. These fibers are widely used as base materials for producing high-performance composite materials. However, PAN carbon fibers are relatively low in elastic modulus which is among the important properties required of starting materials for high-performance composite materials. Those commercially available have mostly an elastic modulus of about 20 to about 30 ton/mm.sup.2 and those with an elastic modulus exceeding 40 ton/mm.sup.2 have not been obtained. The limitation of elastic modulus is attributable to the starting material for producing PAN carbon fibers as known, and is caused inevitably by the crystal growth and crystalline orientation in the interior of carbon fiber. PAN carbon fibers have the further drawback of being costly to manufacture because the carbonization yield is as low as about 50% based on the starting material and stretching is required after completion of the step for infusibility.
On the other hand, pitch-thpe carbon fibers are free of the problems encountered by PAN carbon fibers and thus are expected to be usable as less expensive and high performance materials. Particularly when the fiber is prepared from pitch material hving an optical anisotropy, the crystal growth and crystalline orientation favorably proceed with the rise of temperature (1000.degree. to 3000.degree. C.) in the carbonization of precursor fiber (hereinafter referred to as "pitch fiber"), so that a carbon fiber can be easily produced which has an elastic modulus of 40 ton/mm.sup.2 or higher. The starting material used is a residue produced as a by-product in preparation of other materials for use in some other applications and thus is cheaply available. In addition, the carbonization yield is about 90% based on the weight of pitch fiber. For these reasons, the pitch-type carbon fibers have the advantage of being produced at low costs. However, the pitch which is used as a material for spinning (hereinafter referred to as "spinnable pitch") in the preparation of pitch-type carbon fibers has inherent characteristics: (A) the pitch has an extremely low molecular weight as compared with organic polymers commonly used; (B) the pitch extensively vary in molecular weight and molecular structure, and (C) the melting temperature, i.e. spinning temperature is as high as 300.degree. C. or higher. Accordingly the pitch-type carbon fibers suffer problems different from those involved in use of usual organic polymeric materials. For example, the following problems are posed.
(i) Since a high spinning temperature of molten pitch is required, the viscosity widely varies with the temperature, the strength of pitch fiber if far lower than that of usual organic fibers including PAN, and the pitch-type carbon fibers are inferior in properties of stable continuous spinning to other organic polymers.
(ii) The various aspects of cohesion state of molecules (hereinafter referred to as "high-order sectional structure") can be seen when the section of pitch-type carbon fibers is observed. More specifically, molecules may form crystals along concentric circles (so-called onion-type), molecules may form crystals radially of the axis of fiber (radial-type), molecules may be distributed without orientation (random-type) or the interior layers may be of the random-type and the exterior layers may be of the radial-type. The pitch-type fiber is likely to form microscopic flaws such as cracks, voids and the like. Such high-order sectional structure and flaws greatly affect the dynamic properties of carbon fibers. Particularly the presence of flaws significantly reduces the tensile strength and elongation. The frequency of occurrence of high-order sectional structure and flaws is variable depending on the spinning temperature, shearing stress applied to molten pitch, draft ratio of pitch fiber (winding rate/discharge rate), atmosphere temperature of the relaxing part and insert stretched solidifying part and like usual spinning conditions and properties of spinnable pitch. In view of said variations, these various parameters need to be strictly controlled to make uniform the quality of carbon fibers.
Accordingly in order to produce high-performance pitch-type carbon fibers with stability, the quality-fluctuating factors previously noted need to be eliminated as much as possible. The advent of new technique for fulfilling this objective is strongly desired.